A steam generator used for a pressurized water reactor plant is a heat exchanger including several thousands of heat transfer tubes and a plurality of tube support plates that bundle the heat transfer tubes and have insertion holes formed therein for inserting the heat transfer tubes therethrough. Heat exchange is performed between high-temperature and high-pressure primary cooling water flowing from the reactor into the heat transfer tubes and secondary cooling water flowing outside the heat transfer tubes, and feed water with temperature thereof becoming high due to the heat exchange turns into steam in an upper part of the steam generator and is fed to a steam turbine for power generation.
In a tube support plate, for example, a four-leafed insertion hole (BEC (Broached Egg Crater) hole) is often provided for each heat transfer tube in order to support the heat transfer tube and to distribute the feed water. Chemicals for adjusting the water quality is put in the feed water, and components included in chemicals such as ferrosoferric oxide and water stain may adhere to gaps (portions where an external surface of the heat transfer tube and an internal surface of the BEC hole are away from each other) in the BEC hole to block the flow of the feed water. Therefore, it is desired to evaluate the ratio (blockage rate) of being blocked by scales present in the gaps in the BEC hole and take measures against the blockage such as cleaning as required.
Conventionally, as the method of checking the blockage rate of gaps in a BEC hole, there have been known methods such as a method of taking pictures of a BEC hole by using a CCD (Charge Coupled Device) camera to calculate the blockage rate based on recorded images (see, for example, Patent Literature 1) and a method of estimating the blockage rate based on an eddy current testing signal acquired by using an eddy current testing (ECT). When the method of estimating the blockage rate based on an ECT signal is used, a bobbin-coil eddy-current probe is generally used. An amplitude in a specific direction is measured based on the eddy current testing signal detected by the bobbin-coil eddy-current probe so as to acquire an estimated blockage rate corresponding to the measured amplitude from an evaluation curve.
As a method of evaluating the blockage rate of a BEC hole of a steam generator, first, an actual blockage rate is determined by visual inspection, and an ECT waveform of a gap in a BEC hole at a position (an address) corresponding to the visual inspection is obtained, thereby determining a blockage evaluation value from the ECT waveform. The visual inspection uses an image captured and recorded by the CCD camera or the like. In the present specification, all inspections similar to this visual inspection are described as “visual inspection”. Furthermore, the actual blockage rate read from captured images is described below as “actual blockage rate”. An evaluation curve is created in advance based on the actual blockage rate measured by the visual inspection and the blockage evaluation value based on the ECT waveform. Thereafter, a blockage evaluation value is calculated based on ECT waveforms of a plurality of BEC holes arbitrarily selected from the entire area of the tube support plate, and an estimated blockage rate of a BEC hole is determined by referring to the evaluation curve.
Furthermore, to assess the accumulated position of deposited materials accumulated on a surface of the heat transfer tube and the accumulated amount thereof, there has been proposed a method of schematically imaging a surface of a heat transfer tube to visually assess an accumulation state of deposited materials on the surface of the heat transfer tube (see, for example, Patent Literature 2).